A terahertz electromagnetic wave is an electromagnetic wave having a frequency from 0.1 to 10 THz (wavelength from 30 μm to 3000 μm). This wavelength is substantially the same as a range from the wavelength of a far-infrared wave to that of a millimeter wave. The terahertz electromagnetic wave exists in a frequency range between the frequency of “light” and that of a “millimeter wave.” Thus, the terahertz electromagnetic wave has both an ability to identify an object with a spatial resolution as high as that of light and an ability comparable to that of a millimeter wave to pass through a substance. An electromagnetic wave in the terahertz wave band has not been explored so far. Meanwhile, for example, application of characterization of a material has been examined for time-domain spectroscopy, imaging, and tomography utilizing the characteristics of the electromagnetic wave in this frequency band. The terahertz electromagnetic wave has both the performance of passing through a substance and straightness. Thus, using the terahertz electromagnetic wave instead of an X-ray allows safe and innovative imaging or ultrahigh-speed wireless communication of some hundreds of Gbps.
Veselago showed that incidence of light on a medium having a permittivity and a magnetic permeability both of negative values causes negative refraction and an artificial structure producing a negative permittivity and a negative magnetic permeability has been suggested. Such an artificial structure producing a negative permittivity and a negative magnetic permeability is an artificial structure called a metamaterial having a scale sufficiently larger than atoms and smaller than a light wavelength. Using the metamaterial to cause negative refraction allows formation of a perfect lens having a planar shape. A conventional lens encounters diffraction limitation that makes it impossible to observe an object smaller than the light wavelength. The perfect lens overcomes the diffraction limitation to allow observation of a tiny object.
In one example of a known metamaterial, the metamaterial includes a split ring resonator exhibiting a negative magnetic permeability formed of two rings of two different sizes having respective cuts formed in opposite positions and a matrix of unit cells formed of metallic wires exhibiting a negative permittivity (see patent literature 1). This metamaterial becomes applicable to a lens, for example, by arranging these unit cells along one axis so as to form a gradient refractive index to achieve a negative refractive index.